Nix the idea of getting a boyfriend on the side - that will only add to your problems, not solve them!
As you know alcoholism is a family disease, everyone is affected by the alcoholics behavior. Sometimes the damage is irrepairable, sometimes not. If you are 100% sure that you want out of this marriage then you need to tell him and see an attorney. If there is a part of you that wants to save this marriage then I would start therapy asap and start attending AlAnon meetings.
You've grown apart obviously because he was an active alcoholic. You really need to do some soul searching to find out exactly what you want. Don't be angry at yourself for feeling as you do, your feelings are your feelings. I would seek a therapist alone if you are having a hard time making a decision because any decision that you make will affect your entire family so be sure you are 100% until you take further action.
I've got a feeling that you are holding on to alot of resentments from his "active" drinking years which is understandable. Therapy and AlAnon can help you release that anger so that you can make a decision with a clear head.
